Mina Andreeva, the European Commission president's deputy spokeswoman, categorically denied the report in the Daily Mail.
The newspaper had said Mr Juncker and his second in command, Martin Selmayr, would fly to Ireland for 29 March - the day the UK is set to leave the EU.
It added they would toast to Britain's departure with Leo Varadkar "to show solidarity with the Irish people".
But Ms Andreeva said the story was "simply not true".
"No plan for Jean-Claude Juncker to spend 29 March in Dublin," she said - labelling it a "Brexit untruth".
It follows a tense week for British-EU relations, when EU Council President Donald Tusk said there was a "special place in hell" for Leave campaigners with no plan to carry out Brexit.
Brexit Secretary Stephen Barclay sought to repair them with a trip to Brussels on Monday to discuss a way to break the Brexit deadlock with the EU's chief negotiator.
A UK government spokesperson said they had a "constructive" meeting and that further talks were planned.
Britain is due to leave the EU on 29 March with or without a deal.
Only the government can ask to delay or scrap the departure date.
But it is struggling to get an agreement ratified by MPs that would ensure a more orderly divorce from the 28-member state bloc.
Prime Minister Theresa May is asking Brussels to renegotiate the most controversial aspect, by European leaders say the text agreed back in November cannot be changed.
They are open to updating the text of the political declaration on a future relationship, but some Conservatives fear its non-binding nature will not be enough to give Britain the safeguards it needs.